1. Field of the Invention
The present invention relates to the cooling of turbine rotor components in a gas turbine engine and, more particularly, to a modulating cooling flow through a single circumferential row of inducers for modulating the cooling air flow to such turbine rotor components.
2. Discussion of the Background Art
Gas turbine engines typically include cooling systems which provide cooling air to turbine rotor components, such as turbine blades, in order to limit the temperatures experienced by such components. Prior art cooling systems usually acquire the air used to cool turbine components from the engine's compressor, after which it is diverted and subsequently directed to the turbine section of the engine through an axial passageway. A device commonly known as an inducer is generally located at the exit end of such an axial passageway in order to accelerate and direct the airflow tangential to and in the same direction of the rotating rotor. Such inducers, frequently in the form of a circumferentially disposed array of vanes, are used to control the tangential speed and direction of the airflow so that it is substantially equal to that of the turbine rotor. An exemplary inducer utilized for such purpose is disclosed in U.S. Pat. No. 4,882,902 to James R. Reigel et al., entitled "Turbine Cooling Air Transferring Apparatus". Another inducer performing a similar function to the vane-type inducer is disclosed in U.S. Pat. No. 5,245,821 to Theodore T. Thomas Jr. et al. entitled "Stator to Rotor Flow Inducer," where a plurality of cylindrical airflow passages are disposed circumferentially about the engine centerline and includes cooling airflow holes or passages that are acutely angled in a tangential manner to the rotational direction of the rotor. The passages include a downstream angled outlet in the form of an open channel that is angled in a rotational direction of the rotor and has a back wall that is at a small acute angle with respect to a plane perpendicular to a centerline of the rotor.
An important factor in the design of cooling systems is its relationship to the efficiency of the gas turbine engine. In current prior art systems, the amount of cooling flow is generally fixed at a level required to achieve requisite cooling at the maximum turbine inlet temperature point for the engine. Since an engine is usually run at conditions which are less than maximum turbine inlet temperature, this causes the engine to normally operate with excess cooling flow and decreased efficiency. This excess cooling also has the effect of increasing overall engine specific fuel consumption. Accordingly, an apparatus capable of modulating the flow of cooling air through an inducer to the turbine rotor in accordance with the engine cycle requirements would increase the efficiency of the gas turbine engine and be most desirable.
Modulated turbine rotor cooling systems incorporating such inducers are disclosed in U.S. Pat. Nos. 4,462,204, 4,807,433, and 5,575,616, which provide both a modulated circumferential row and unmodulated circumferential row of inducers disposed radially inward and outward of each other respectively. These systems, however, are inefficient because they modulate the airflow through two different circumferential rows of inducers which adds to construction costs and design complexity. The inner and outer circumferential rows of inducers have different flow conditions and face different tangential velocities of the rotor at their exits. This results in inefficiency and compromises in design which lowers the overall efficiency of the modulated turbine rotor and inducer cooling system. The cooling system in U.S. Pat. No. 4,807,433 uses the same source of air with radially inner and outer plenums and inducers. The radially inner and outer inducers would have different dimensions and shapes. It is difficult to design these inducers to avoid mismatch of airflow conditions along the boundary of the cooling air exiting the inducers. This in turn could cause turbulence along the interface of the two exit airflows which in turn would reduce the efficiency improvement which the invention is designed for. This problem becomes more significant as the engine is operated farther away from design conditions. Consequently, there exists an unfulfilled need for an apparatus which modulates the flow of cooling air to the turbine at the exit of the cooling flow system in a more efficient and cost effective manner. The present invention address this need, in its preferred embodiment, by providing a single circumferential row of inducer passages having a portion of inducer passages fed by an independently modulated source of cooling air.